Liquid crystal display device is generally composed mainly of a liquid crystal cell having a liquid crystal layer sandwiched between a pair of substrates, a retardation film and a pair of polarizing films arranged perpendicular to each other and placed at both sides of the liquid crystal cell. Liquid crystal display devices have been used in wide fields from small-sized instruments such as watch and electronic calculator to large-sized instruments such as monitor and television set. Elements of TN (Twisted Nematic) mode to use a liquid crystal molecule having positive dielectric anisotropy were the mainstream in such liquid crystal display devices. In a TN-mode liquid crystal display device, the orientation direction of the liquid crystal molecule adjacent to one substrate is twisted by about 90° relative to the orientation direction of the liquid crystal molecule adjacent to the other substrate in a state free from the application of voltage.
Various developments were performed on the TN-mode liquid crystal display devices for realizing good black display and high contrast. It is necessary in the TN mode to display the black color under the application of voltage, i.e. orient the major axis of the liquid crystal molecule in the direction nearly perpendicular to the panel face. However, since the liquid crystal molecule adjacent to a liquid crystal panel substrate keeps horizontal orientation in the TN mode even under the application of voltage, the polarized state of light varies by the birefringence of the liquid crystal molecule. Consequently, complete black color cannot be displayed even by viewing the panel from perpendicular direction to make the realization of high contrast difficult.
On the contrary, in a liquid crystal display device of vertical orientation mode, so-called VA (vertical aligned) mode, the major axis of a liquid crystal molecule is oriented nearly perpendicular to the panel face in a state free from the application of voltage between a pair of substrates constituting the liquid crystal panel. In the vertical aligned mode, the liquid crystal molecule adjacent to the liquid crystal panel substrate also takes a nearly perpendicular orientation relative to the panel face and, accordingly, the polarization state of the light is scarcely varied by the transmission of the light through a liquid crystal layer. Consequently, a nearly complete black display superior to TN mode usually becomes possible to realize high contrast when the cell is viewed perpendicular to the substrate.
Various techniques have been proposed for improving the view angle of conventional VA mode display. For example, the specification of the JP-A 11-95208 (hereunder JP-A means “Japanese Unexamined Patent Publication) describes a vertical aligned nematic liquid crystal display device composed of a liquid crystal cell, a pair of polarizing films placed above and below the liquid crystal cell in a state directing the absorbing axes of the films perpendicular to each other, and a retardation film for the compensation of view angle composed of one or two retardation films placed between the liquid crystal cell and at least one of the polarizing films. Concretely disclosed technique is the use of one retardation film or two laminated retardation films placed between the liquid crystal cell and one of the polarizing films (refer to the claim 1 and the Examples 1 to 4 (sections 0030 to 0034).
The specification of JP-A 2000-131Σdiscloses a VA-mode liquid crystal display device produced by inserting a specific biaxial retardation film between a substrate and a polarizing film in a manner to direct the slow axis in the plane of the retardation film to be nearly parallel or perpendicular to the absorption axis of said polarizing film placed at the side of the retardation film relative to the liquid crystal layer. Concretely, a lamination of two specific retardation films (refer to section 0064, FIG. 54) or two specific retardation films placed at both sides of a liquid crystal cell (section 0070, FIG. 60) are described in the specification.
However, the VA mode liquid crystal display devices described in these specifications were successful for the improvement of the view angle characteristics only at a specific wavelength. In other words, the transmission of the light of a specific wavelength is decreased by slantly viewing the liquid crystal display device displaying black color resulting in the widening of the view angle. However, a problem of the coloring of black color owing to the leakage of light occurs in these cases when the wavelength of the light is different from the specific wavelength.
The main object of the invention is to provide a new liquid crystal display device of VA mode.
Another object of the invention is to provide a VA mode liquid crystal display device causing little leakage of light over the whole visible light range in the case of displaying black color and enabling the display of nearly achromatic black color.
A further object of the invention is to provide a new method for suppressing the light leakage over the whole visible light range to enable the display of nearly achromatic black color by using a retardation film in a VA mode liquid crystal display device in the case of displaying black color.
Still further objects and advantages of the invention will become apparent from the following explanation.